As per the Account setup, Forums form, I have adjusted the explanation text to better indicate what the use of the "Automatic reset of new messages" serves for. Basically, this is the text:

Check each forum you wish to subscribe to. The "Automatic reset of new messages" allows you to have new messages to become old messages as soon as you refresh to get them. Otherwise, the related messages will remain there when you refresh until you click on the reset icon next to the forum title.

So, this is much better now to reset the new messages of a forum which is not set for automatic reset. Just a simple click on the icon next to the forum title would do.